Reversible inhibition by pyridoxal 5'-phosphate and irreversible inactivation by urea and guanidine hydrochloride of thymidylate synthase from Lactobacillus leichmannii.

Pyridoxal 5'-phosphate reversibly inhibited thymidylate synthase from Lactobacillus leichmannii. The inhibition was competitive with dUMP (Ki = 1 microM) and non-competitive with 5,10-CH2-THF (Ki = 0.08 microM). Treatment of native or pCMB-treated enzyme with urea (5 M) or guanidine hydrochloride (4 M) resulted in inactivation and dissociation of the homodimer (74 kDa) into monomer (37 kDa).